1st law of motion

"A body tends to be in a state of rest or of uniform motion until an external force is applied on it."

A frictionless inclined plane with same angle

This law was given by SIR ISSAC NEWTON, however it was first observed by GALILEO GALILEI. He(galileo galilei) performed an experiment where in he took frictionless 2 frictionless surfaces or planes namely 1 & 2 inclined at the same angle from the ground facing each other as shown in the fig 1 . He then dropped a smooth frictionless ball from one end of the planes noting it's height and he saw that the ball is traveling to the same height on the other plane also. He then changes the angle of the plane 2 and repeated the same procedure with varying the angle of plane 2. Each time he performed the experiment he observed that the ball is trying to attain the same height from which he dropped the ball. then what he did was that, he places a frictionless surfaced on the ground as shown in fig. 2 and dropped the ball from the same height. He saw that the ball is going on traveling it did not stop. For which he concluded that "the ball keeps on traveling as it is trying to attain the height from which it was dropped which is not possible as the angle between the ground and the plane is 0, so the ball will keep on traveling trying to attain this height." Later NEWTON studied Galileo's work and came up with this 1st law of motion.

Inclined planes with different angles

Where do we see the 1st law of motion coming in application?

When we are sitting in the bus or car and the driver suddenly applies brakes and we fall forward.